A conventional motion transmission system comprises, in the rear part of the bicycle, a wheel equipped with a hub comprising a body element, in the jargon known as “freewheel body”, capable of rotating idly with respect to the hub in one direction of rotation and of making it rotate in the opposite direction.
The freewheel body is generally cylindrical with outer grooves adapted to engage a plurality of sprockets of different diameter forming part of a so-called sprocket assembly. In particular, the sprockets are mounted by arranging substantially annular spacer elements between them. Such spacer elements are fitted onto the freewheel body and are active at the radially inner end portions of the sprockets.
The sprockets are in turn, from time to time, according to the transmission ratio selected by the cyclist, engaged and made to rotate by a chain, which receives the movement from at least one guide toothed wheel associated with a crank arm.
Whereas the spacer elements are often made from light metallic material (for example aluminum) or composite material (for example comprising structural fibers incorporated in a matrix of polymeric material), the sprockets are often made from an extremely strong and consequently heavy material, like steel or titanium, to withstand the wear of the chain and the loads to which they are subjected in operation.
Consequently, due to the weight of the aforementioned sprockets, the weight of the sprocket assembly is generally high.
Some known sprocket assemblies have sprocket support elements made from light material, a sprocket being coupled at opposite sides to each of the sprocket support elements. The two sprockets supported by each support element are in abutment with opposite contact areas of the support element. Such contact areas are arranged at a predetermined radial distance with respect to a longitudinal rotation axis of the freewheel body.
In this sprocket assembly, the sprockets are substantially circular crown shaped and are not coupled with the aforementioned freewheel body. The support elements comprise a substantially annular portion, provided with a radially inner toothing adapted to engage the grooves of the freewheel body of the hub. Such support elements are made from light material, like aluminum.